Sheets of metal (which can include strips of metal) are coated with a material to provide the metal with certain desired physical properties. For example, sheets of steel are coated with a protective zinc-based material in a process known as galvanization. The protective coating inhibits the sheet from oxidizing. Galvanized sheet metal is used for automotive applications, such as car door panels.
In a conventional galvanization process line, a long sheet/strip of metal is drawn off a reel and continuously fed, using a series of rollers and guides, through a bath of molten zinc-based material. Molten zinc-based coating is typically reflective. Upon emerging from the bath the coated portion of the sheet passes between air knives. The air knives expel high pressure/velocity air onto both sides of the sheet to force excess molten coating off the sheet and preferably back into the bath.
It is important to precisely control the amount or thickness of zinc-based coating on the metal sheet. It is also important to limit the defects/imperfections in that coating. The coating can be very expensive. Too much coating increases material costs with potentially no functional benefits or way to recoup such costs in the market. Too little coating, or imperfections/defects in the coating, may result in the sheet not meeting certain industry specifications, and may prevent selling the sheet at a premium price. This is because too little coating and/or imperfections/defects in the coating could cause the sheet to be more susceptible to oxidation, or affect the appearance of a subsequently painted panel or product using the sheet, thereby limiting the applications for which it can be used and the lifetime of the sheet.
The proximity of the air knives to the sheet is one of the main parameters used to control the coating thickness. The metal sheet can, however, buckle, warp, bow, and tremble around the area of the air knives as the sheet continuously passes between the air knives. This can cause rapid changes in the distance between each air knife and the sheet, resulting in variations in coating thickness, and potentially imperfections in the coating surface. Typically, the closer the air knives are to the sheet, the better control there is over coating thickness. But the closer the air knives are to the sheet, the greater the risk that sheet movement/deformation will result in the sheet hitting an air knife. This can block the air knife vents with coating material, potentially damage the air knife, introduce imperfections into the coating, and potentially require the shutdown of the entire galvanization line.
Methods have been proposed to reduce unintended variations in the distance between the air knifes and the sheet by detecting a change in the desired distance and rapidly compensating for that change. In a method, an ultrasonic sensor is used to sense the distance between the air knife and sheet, and the air knife and sheet are moved relative to each other to compensate for variations in the desired distance. However, ultrasonic sensors can fail to accurately report a distance to a hot surface. In another method, an induction sensor is used to sense the distances between the air knifes and the sheet, and corrections are made to compensate for detected distance variations. However, induction sensors have a limited sensing range and do not cover the full scope of operating conditions. An inductive sensor with a sufficient range would be excessively large and highly nonlinear.
A reliable method of detecting the distance to a reflective surface is desirable. More specifically, a reliable method of detecting the distance from an air knife to a reflective zinc coated surface in a continuous coating process is desirable.
Imperfections can also occur in the coating of a surface. In a sheet metal galvanization process, imperfections may affect a section or area of the coating on the metal sheet. The actual size of each imperfection may be quite small (such as less than 0.1 millimeters), but the overall affected section or area may be quite large. The cause of imperfections may vary. Camera-based surface inspection systems are sometimes used to detect certain types of defects by taking a picture of the sheet and trying to identify defects based on the aggregate picture data through image recognition algorithms, for example. Such systems are typically located, however, much further down the line from the metal bath coating stage where the problem originates. This can result in a considerable amount of product being processed by the line before a problem is detected.
A method for identifying the existence of imperfections, and the extent or severity of the imperfections, is desirable. A method for early detection of imperfections in the coating process is also desirable so the cause can be more easily identified and more quickly remediated.